Optogenetic actuators are ion channels or pumps that can be regulated by light, thus permitting neuronal activity to be turned on and off with high spatial and temporal precision. Optogenetics holds significant promise for restoring vision to blind patients, but current treatment strategies require the application of high-intensity blue-green light, which poses a significant risk of retinal photodamage. Dependence on the use of short-wavelength light therefore represents a major barrier to safe and effective implementation of optogenetic therapy for retinal disease. This barrier can be surmounted by the use of optogenetic actuators with red-shifted excitation spectra. Red light is less energetic and therefore less damaging to the retina. Accordingly, researchers have sought to develop red-shifted optogenetic actuators, and considerable progress has been made in red-shifting actuators via opsin engineering. However, in order to extend the operational range of optogenetic actuators into the near-infrared (>700 nm), new orthogonal approaches are needed. The goal of the present proposal is to introduce a novel biomimetic strategy for red-shifting optogenetic actuators: red- shifted chromophore substitution. This approach is complementary to opsin engineering and is based on a strategy used by migrating fish to enable better vision in turbid water. When salmon migrate from the open ocean into inland streams (where incident light is significantly red-shifted), they switch from using retinal as their visual chromophore to , 4-didehydroretinal which has red-shifted spectral properties. This chromophore switch causes a dramatic red-shift of the fish's opsin spectral sensitivity, thereby enhancing the animal's abilityto see long-wavelength light and thus permitting the animal to peer more deeply into turbid streams. Our goal is to identify the enzyme mediating the conversion of retinal into 3, 4-dodehydroretinal, and to co-express it with optogenetic actuators in mammalian neurons in vivo, thereby red-shifting their action spectra. In Specific Aim 1, we use transcriptome profiling in zebrafish and bullfrog to identify the enzyme mediating this conversion, and then characterize its function in vivo. In Aim 2, we will co-express this enzyme with red-shifted optogenetic actuators in vivo to endow non-functioning mouse photoreceptors with sensitivity to near-infrared light (>700 nm). A key feature of this approach is that chromophore substitution can be coupled to the use of any existing actuator in any part of the mammalian nervous system. Thus, this proposal promises to have a widespread impact on the field of optogenetics.